Why are soccer fans more violent than rugby fans?

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I have noticed with great alarm the violence related with soccer. I am a rugby fan and have never seen any violent behaviour at a rugby match. I was in my local golf club yesterdy watching South Africa playing England with a mixture of South African and English fans, the haggling was pretty good but no sign of violent behaviour. I have heard a saying which goes something like "Soccer is a game played by gentlemen and watched by hooligans but Rugby is a game played by hooligans and watched by gentlemen." Rugby is a hard physical game but I don't see a lot of hooligans playing the game, in fact most rugby players are highly educated people. Why are soccer fans so violent and rugby fans not?